How much difference does a phono stage make?

Agree with all the above.

I've owned $150 cartridges and $5,000 cartridges. I also own an $1,800 preamp and a $12,000 one. Like Syntax, if I had to pair the lesser of one with the greater of the other - and could only keep one combo - I'd keep the good preamp + a cheap cartridge, not the cheap preamp + a good cartridge.

The same is true of turntables and tonearms vs. cartridges. If I had to choose between my $1K rig + a $5K cartridge or my $11K rig + a $150 cartridge, I'd choose the latter every time. $150 cartridges play better than you'd think possible on a good rig. $5K cartridges on a cheap rig just reveal its flaws and shortcomings.

In general, the expenditure or upgrade order should be:
1. phono stage
2. turntable
3. tonearm
4. cartridge

In your case, your next upgrade after the phono stage should be speed control for the Scout, which will also make a nice improvement.

The PH-3 is a very good phono preamp. It is an older model(not the lowest end model) It was at one point ARC's top of the line model.(model years 1995-1998) You will notice a significant difference in performance from what you have now!
For those who don't know, the line of progression of ARC phono preamps: PH-1 and the Reference models: Reference 1
PH-2 Reference model 2
PH-3
PH-3Se
PH-5
PH-7

Totally agree w all posters above.

Look for the PH3SE, it is worth the $150 more difference. I had the PH3 and then the PH3SE. Plenty of difference. Great value. The 5 and 7 are also good but not good values.

the difference will be night and day, entirely worthwhile. However, it may reveal the limitations of other parts of the chain that you were not aware of.